1. Field of the Invention
This invention relates to apparatus for processing grain-oriented electrical steel strip which comprises a core plating line of a conventional type, in which a thin layer of insulation coating is formed on the surface of the strips, and a core-loss improving device that are disposed so that either or both of a common core plating process and a core-loss improving process can be selectively applied to the strip as desired.
2. Description of the Prior Art
As is widely known, grain-oriented electrical steel strip covered with a glass film formed by final (texture) annealing is commonly finished in a coating and flattening line where an insulation coating is applied and baked. Spurred by the recent trends to seek as much energy saving as possible, needs for core loss improvement have increased steadily and several technologies to fulfill such needs have been completed and patented. One of them reduces domain size by use of laser beams. These types of technologies have achieved remarkable core loss improvements with grain-oriented electrical steel strip for stacked cores to which stress-relief annealing is not applied. Inventions on them are disclosed, for example, in the U.S. Pat. No. 4,468,551, Japanese Patent Publications Nos. 2252 of 1982, 36051 of 1983, 50298 of 1983, and so on.
On the other hand, the U.S. Pat. No. 4,770,720 discloses a domain size reducing technology that has proved remarkably effective for grain-oriented electrical steel strip for wound cores that is stress-relief annealed. This method comprises scribing grooves in the surface of the electrical steel strip, with a force of 90 to 220 kgf/mm.sup.2, set at an angle of 45 to 90 degrees with the rolling direction and then applying a heat treatment at a temperature of 750.degree. C. or above. The grooves can be scribed by use of toothed-wheel-like rolls whose teeth extend in the direction of, or parallel with, the roll axis, as disclosed in the Japanese Provisional Patent Publication No. 15314 of 1986, the U.S. Pat. No. 4,533,409, and so on.
Because of the severe vibrations set up in scribing, however, the grooves formed by this type of toothed-wheel-like rolls tend to become irregularly spaced over the breadth of the strip, thereby inducing considerable core loss variations. Scribing rolls with helically or diagonally cut teeth of the type disclosed in the Japanese Provisional Patent Publication No. 15314 of 1986 can offer solution for the above problem.
And yet, another problem comes up with helically toothed rolls. When the electrical steel strip is passed, with a given draft, between a helically toothed roll and a pressing roll thereunder, a force set up between the helically toothed roll and the strip works at right angles with the direction of strip travel (or in the direction of the roll axis). The force tends to cause the strip to move breadthwise, thereby hampering the smooth travel of the strip.
The inventor has already proposed a technology to overcome this difficulty in an invention disclosed in the Japanese Provisional Patent Publication No. 153222 of 1988. As is described in the publication, the proposed method provides a roll (hereinafter called the bridle roll) above a toothed roll. The strip fed through a horizontal pass line travels forward to a groove-scribing unit via the bridle roll, with the access angle of the strip reaching the bridle roll adjusted by a preceding roll.
The core-loss improving groove scribing unit and the heat treatment line for the grain-oriented electrical steel strip for wound cores may be installed separately. The insulation coating on the ordinary grain-oriented electrical steel strip is baked at a temperature of 750.degree. C. or above in the coating and flattening line. On the other hand, an insulation coating must be formed on the grain-oriented electrical steel strip covered with a glass film formed by final annealing and scribed with core-loss improving grooves. As such, installing a groove-scribing unit of the type disclosed in the Japanese Provisional Patent Publication No. 153222 of 1988 on the entry side of an insulation coating unit of the type incorporated in the conventional coating and flattening lines for grain-oriented electrical steel strip offers considerable operational advantage by permitting sharing of the insulation coating unit and its auxiliary facilities.
When only an ordinary insulation coating is needed, the grain-oriented electrical steel strip covered with a glass film insulation is passed direct to, i.e., not by way of the bridle roll, the insulation coating unit along the horizontal pass through the open groove-scribing unit. When a core-loss improving treatment is needed, the strip is passed first over the bridle roll to assure a stable travel, and then to the groove-scribing unit, where grooves extending in the longitudinal direction are scribed in the surface of the strip, and to the insulation coating unit to form an insulation coating in the course of a heat treatment that is applied at a temperature of 750.degree. C. or above.
If the bridle roll is fixed as in the preferred embodiment disclosed in the Japanese Provisional Patent Publication No. 153222 of 1988, however, the following steps must be taken for switching the pass line from a horizontal one to a detour pass line via the bridle roll in the course of the continuous strip travel to switch from the ordinary insulation coating process to the core-loss improving process and vice versa.
When switching to the core-loss improving process is anticipated, the grain-oriented electrical steel strip is passed through the line with a lead strip (which will not constitute a portion of the finished product) connected to the leading end of the strip to be processed. When the lead strip has been passed over the entire length of the line, the line is stopped and the temperature of the baking furnace in the insulation coating unit is lowered. Then, the strip is cut on the entry side of the groove-scribing unit. Next, the leading end of the downstream strip is passed over the bridle roll and connected to the tail end of the upstream strip, thus forming a detour pass line. Then, the temperature of the baking furnace is raised to the desired level and the line is started again.
But this method has the following shortcomings.
(1) Stopping the line significantly lowers productivity (tonnage output per hour).
(2) Lowering and raising the baking furnace temperature entails additional energy cost.
(3) Cutting and connecting the strip is an elaborate job requiring additional manpower.
(4) The use of the lead strip pushes up the operation cost.